The present invention relates to an actuation circuit for a switch regulating the power consumption in a switching converter. In one case, a switching converter is configured as a buck converter.
Buck converters are switching converters (switched-mode converters) used to convert an input voltage into an output voltage that is lower in comparison to the input voltage. The fundamental layout of such buck converters is described, for example, in Stengl, J. P.; Tihanyi, J.: “Power MOS-FET Practice”, 2nd Edition, Pflaum Verlag, Munich, page 176, or in Tarter, R. E.: “Solid-State Power Conversion Handbook”, Wiley & Sons, New York, 1993, ISBN 0-471-57243-8, pages 350, 351.
An important element in such a buck converter is an inductive storage element, which is connected to a supply voltage and timed by a pulse width modulated actuating signal.
In order to regulate the output voltage to an approximately constant value, not depending on the current consumption of a load connected to the output terminals, a regulating arrangement is present in such switching converters, which detects the output voltage or the changes in the output voltage. If the output voltage deviates from a given nominal value, the power consumption of the buck converter is changed by changing the on-period, in order to regulate the output voltage back to the nominal value once again. For buck converters that work in so-called current mode control, in order to generate the pulse width modulated actuating signal, one uses the time course of the current across the inductance to generate the actuating signal. This time function, which approximately corresponds to a triangular signal function, is compared to a regulating signal that depends on the output voltage in order to determine the time for the start or end of an on-period. Either the end or the start of the on-period will usually be assigned a fixed time.
A buck converter with current mode operation is described, for example, in Tarter, op.cit., pages 492 to 495.
In such current mode buck converters (CM buck converters), the measuring of the current across the inductance causes problems. In particular, it is difficult to detect a current for which a measurement signal is available both when the switch is closed and when the switch is open, that is, during the entire course of an actuation period of the switch.
To measure the current across the inductance, one can use a measuring resistor connected in series with the inductance, and pick off the voltage across the resistor. However, the voltage across the resistor is related to a potential which varies greatly according to the operating state of the switch, which makes it difficult to evaluate the voltage across the shunting resistor. Furthermore, the power dissipated by this sense resistor is proportional to the square of the load current, therefore the overall efficiency of the converter is severely compromised at high currents.